U.S. Pat. No. 3,620,816, which issued to the same individuals named as applicants herein, describes a process of diffusion alloying the surface of a ferrous part in a molten lead medium. In that process, elements such as chromium are dissolved into the molten lead, and the lead is placed in contact with the ferrous part to be surfaced. The diffusing element is alloyed into the surface of the part by metallic diffusion at elevated temperatures. The process has been applied simultaneously to multiple small parts where bonding of the parts must be avoided. U.S. Pat. No. 3,778,299, which also issued to the same individuals named as applicants herein, describes a method to avoid bonding in the processing of multiple small parts.
There are situations where bonding of parts is necessary. Such parts may be bonded together by any of several well-known methods, such as by brazing or self-interdiffusion. In the latter process, heat will cause the surface of one component to diffuse into the other, and vice versa. If, in addition, it is desired to have a diffused layer in these parts, a second separate operation would be necessary.
Fabrication of honeycomb structures presents special problems in bonding. Such stuctures may be made by utilizing alternate layers of corrugated sheet and uncorrugated (or less corrugated) sheet. Alternatively, the corrugated and uncorrugated sheets may be simultaneously wrapped to provide interposed helical configurations. Either structure results in the creation of a multiplicity of parallel channels. In certain uses of such honeycomb structures, it is important that the uncorrugated sheet be securely bonded to the corrugated sheet at the lines of contact between the two sheets to cause fluid to flow only axially through the channels and preclude fluid flow transverse thereto. For example, such honeycomb structures may be used in energy-saving heat regenerator wheels or as substrate carriers for catalytic converters. In other applications, transverse fluid flow is acceptable. In either case, it is important that the product be unitary by way of substantial bonding throughout.
Such structures may be made of stainless steel sheet, in order to withstand the high temperatures to which they may be subjected. However, it is difficult to form stainless steels and other high temperature alloys very thin (e.g., gauges of two mils.) and then corrugate. It is also difficult to obtain consistent bonding throughout the product. As explained above, this is particularly significant where transverse fluid flow must be avoided.